Upon oxidation, a silica scale forms on MoSi2, a potential high-temperature coating material for metals. This silica scale protects MoSi2 against high-temperature corrosive gases or liquids. We use periodic density fu...
Upon oxidation, a silica scale forms on MoSi2, a potential high-temperature coating material for metals. This silica scale protects MoSi2 against high-temperature corrosive gases or liquids. We use periodic density functional theory to examine the interface between SiO2 and MoSi2. The interfacial bonding is localized, as evidenced by an adhesion energy that changes only slightly with the thickness of the SiO2 layer. Moreover, the adhesion energy displays a relatively large (0.40J∕m2) variation with the relative lateral position of the SiO2 and MoSi2 lattices due to changes in Si−O bonding across the interface. The most stable interfacial structure yields an ideal work of adhesion of 5.75J∕m2 within the local density approximation (5.02J∕m2 within the generalized-gradient approximation) to electron exchange and correlation, indicating extremely strong adhesion. Local densities of states and electron density difference plots demonstrate that the interfacial Si−O bonds are covalent in character. Mo−O interactions are not found in the SiO2∕MoSi2 interface investigated here. Our work predicts that the SiO2 scale strongly adheres to MoSi2, and further supports the potential of MoSi2 as a high-temperature structural material and coating.
It has recently been shown that triply periodic two-phase bicontinuous composites with interfaces that are the Schwartz primitive (P) and diamond (D) minimal surfaces are not only geometrically extremal but extremal f...
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It has recently been shown that triply periodic two-phase bicontinuous composites with interfaces that are the Schwartz primitive (P) and diamond (D) minimal surfaces are not only geometrically extremal but extremal for simultaneous transport of heat and electricity. The multifunctionality of such two-phase systems has been further established by demonstrating that they are also extremal when a competition is set up between the effective bulk modulus and electrical (or thermal) conductivity of the bicontinuous composite. Here we compute the fluid permeabilities of these and other triply periodic bicontinuous structures at a porosity ϕ=1∕2 using the immersed-boundary finite-volume method. The other triply periodic porous media that we study include the Schoen gyroid (G) minimal surface, two different pore-channel models, and an array of spherical obstacles arranged on the sites of a simple cubic lattice. We find that the Schwartz P porous medium has the largest fluid permeability among all of the six triply periodic porous media considered in this paper. The fluid permeabilities are shown to be inversely proportional to the corresponding specific surfaces for these structures. This leads to the conjecture that the maximal fluid permeability for a triply periodic porous medium with a simply connected pore space at a porosity ϕ=1∕2 is achieved by the structure that globally minimizes the specific surface.
The chemical mechanisms underlying the growth of cave formations such as stalactites are well known, yet no theory has yet been proposed which successfully accounts for the dynamic evolution of their shapes. Here we c...
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The chemical mechanisms underlying the growth of cave formations such as stalactites are well known, yet no theory has yet been proposed which successfully accounts for the dynamic evolution of their shapes. Here we consider the interplay of thin-film fluid dynamics, calcium carbonate chemistry, and CO2 transport in the cave to show that stalactites evolve according to a novel local geometric growth law which exhibits extreme amplification at the tip as a consequence of the locally-varying fluid layer thickness. Studies of this model show that a broad class of initial conditions is attracted to an ideal shape which is strikingly close to a statistical average of natural stalactites.
In this paper, we consider a ranging procedure occurred in the fixed and mobile broadband wireless access systems based on the Wireless MAN-OFDMA (Metropolitan Area Network -Orthogonal Frequency Division Multiple Acce...
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In this paper, we consider a ranging procedure occurred in the fixed and mobile broadband wireless access systems based on the Wireless MAN-OFDMA (Metropolitan Area Network -Orthogonal Frequency Division Multiple Access). A ranging procedure is a collection of processes for a Mobile Subscriber Station (MSS) and a Base Station (BS) to maintain the quality of the radio frequency communication link between them. When multiple subscribers select the same ranging codes from the given ranging code pool and range the BS by transmitting those identical ranging codes at the same time frame, the ranging collision may be occurred at BS because the BS can not differentiate two different MSSs since they selected the same ranging code. We propose a mathematical performance model to analyze the probability of ranging collision affected by both the capacity of the ranging code pool and the rate of the MSS arriving to the system and investigate the numerical results of the proposed model. By the proposed model and its numerical results, we can calculate the minimum number of the required ranging codes to be implemented in BS(Base Station). We compare the analytical results with the simulated results to validate the proposed mathematical model is correct.
Recent simulations indicate that ellipsoids can pack randomly more densely than spheres and, remarkably, for axes ratios near 1.25∶1∶0.8 can approach the densest crystal packing (fcc) of spheres, with a packing frac...
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Recent simulations indicate that ellipsoids can pack randomly more densely than spheres and, remarkably, for axes ratios near 1.25∶1∶0.8 can approach the densest crystal packing (fcc) of spheres, with a packing fraction of 74%. We demonstrate that such dense packings are realizable. We introduce a novel way of determining packing density for a finite sample that minimizes surface effects. We have fabricated ellipsoids and show that, in a sphere, the radial packing fraction ϕ(r) can be obtained from V(h), the volume of added fluid to fill the sphere to height h. We also obtain ϕ(r) from a magnetic resonance imaging scan. The measurements of the overall density ϕavr, ϕ(r) and the core density ϕ0=0.74±0.005 agree with simulations.
A general technique for the generation of canonical channel models and demonstrate the application of the technique to time-frequency and time-scale integral kernel operators is developed. As an example, the derivatio...
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Suspensions of aerobic bacteria often develop flows from the interplay of chemotaxis and buoyancy. We find in sessile drops that flows related to those in the Boycott effect of sedimentation carry bioconvective plumes...
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Suspensions of aerobic bacteria often develop flows from the interplay of chemotaxis and buoyancy. We find in sessile drops that flows related to those in the Boycott effect of sedimentation carry bioconvective plumes down the slanted meniscus and concentrate cells at the drop edge, while in pendant drops such self-concentration occurs at the bottom. On scales much larger than a cell, concentrated regions in both geometries exhibit transient, reconstituting, high-speed jets straddled by vortex streets. A mechanism for large-scale coherence is proposed based on hydrodynamic interactions between swimming cells.
A simple model displaying structural-acoustic behavior is considered. The model comprises of two parallel infinitely-long flat membranes lying on elastic foundations and the acoustic medium which separates them. The s...
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A simple model displaying structural-acoustic behavior is considered. The model comprises of two parallel infinitely-long flat membranes lying on elastic foundations and the acoustic medium which separates them. The structural-acoustic coupling is demonstrated by the fact that a vibrational excitation of one of the membranes triggers vibrations in the other. Being two-dimensional and involving a small number of parameters, the proposed model is especially simple - however, at the same time, it also exemplifies some important features associated with structural-acoustic coupling. Hence, it may serve as a benchmark for the evaluation of numerical structural-acoustic schemes and, possibly, as an educational tool for studying structural-acoustic coupling in a simple context.
It is shown that sigma-delta (/spl Sigma//spl Delta/) algorithms can be used effectively to quantize finite frame expansions for R/sup d/. Error estimates for various quantized frame expansions are derived, and, in pa...
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It is shown that sigma-delta (/spl Sigma//spl Delta/) algorithms can be used effectively to quantize finite frame expansions for R/sup d/. Error estimates for various quantized frame expansions are derived, and, in particular, it is shown that /spl Sigma//spl Delta/ quantizers outperform the standard PCM schemes.
We provide methods of computing multivalued solutions to the Euler-Poisson system and test them in the context of a klystron amplifier. An Eulerian formulation capable of computing multivalued solutions is derived fro...
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We provide methods of computing multivalued solutions to the Euler-Poisson system and test them in the context of a klystron amplifier. An Eulerian formulation capable of computing multivalued solutions is derived from a kinetic description of the Euler-Poisson system and a moment closure. The system of the moment equations may be closed due to the special structure of the solution in phase space. The Eulerian moment equations are computed for a velocity modulated electron beam, which has been shown by prior Lagrangian theories to break in a finite time and form multivalued solutions. The results of the Eulerian moment equations are compared to direct computation of the kinetic equations and a Lagrangian method also developed in the paper. We use the Lagrangian formulation for the explicit computation of wave breaking time and location for typical velocity modulation boundary conditions.
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